High-voltage systems of electrically assisted or fully electrically driven vehicles require monitoring in terms of their insulation level. The voltages on the battery side have a level of up to several 100 volts, namely the HV range (high-voltage range). The batteries in the HV range are coupled to a high-voltage component in the form of current distribution via HV contactors. The current distribution acts on the converters, for example, for the drive machines in the drive or generator mode. In turn, in the high-voltage range the converters apply alternating current (three phases) to the corresponding machines.
In addition, the on-board power supply system designed for the low-voltage range (12V, 24V) is also supplied via a DC-to-DC converter. The high-voltage electrics and the low-voltage electrics are in this case DC-isolated from one another. All of the lines in the HV range (i.e., both the positive and the negative line on the DC side) are insulated with respect to the vehicle earth, the bodywork.
The insulation and line configuration is such that an insulation fault in a line or a terminal does not directly result in any risk, i.e., in a hazardous touch voltage between the housing and earth. Such a first insulation fault does not bring about a short circuit of the battery terminals either. This is only the case when the second line also has an insulation fault. In order to locate such insulation faults as early as possible, insulation detection systems are provided which, as an on-board device, constantly check the insulation state and indicate it to the driver or initiate safety measures.
DD 279 958 A1 describes, in the case of an electrically driven vehicle, a circuit arrangement for monitoring the insulation state of a fraction circuit with respect to other vehicle parts. What is provided is an AC voltage generator which is adjacent to the vehicle earth on one side and has an internal resistance which is matched to the minimum permissible insulation resistance. The AC voltage generator is connected to the traction circuit to be monitored via capacitor circuits. The output is connected to a threshold value switch. The threshold value switch responds to an undershoot of the AC voltage.
DE 43 39 946 A1 describes a method for insulation monitoring of unearthed DC systems. Alternately, different pulse voltage values are applied as measurement voltage via resistive system coupling. The measurement current produced in the settled state is detected. In order to determine the ohmic insulation resistance, differences between successive measurement current values are used.
A circuit arrangement for determining the insulation resistance of a rechargeable battery is described in DE 196 1897 B4. The no-load voltage and the load voltage is measured, for which purpose two capacitors are connected in series between the battery terminals. The common point of the capacitors is connected to earth via a bleeder resistor. Measurement amplifiers are connected by switching means for predetermined times for measuring the partial voltages set across the capacitors.
A circuit arrangement for insulation and contactor monitoring of the power supply of an electric drive is disclosed in DE 10 2006 050 529 A1. High-voltage battery and power electronics are connected to one another via contactors. A voltage source connected to the vehicle earth produces a measurement voltage. Two voltmeters measure the voltage steps produced by the measurement voltage with respect to the vehicle earth. The voltage source couples the measurement voltage onto the line via measuring resistors in such a way that at least one contactor is connected between the feed point and the measurement point.
In order to check the contact between a high-voltage battery and the lines of a high-voltage system, DE 10 2008 012 418 A1 proposes a capacitor which is arranged in parallel with the load and which is charged via a precharging resistor and is discharged to a prescribed relatively low voltage value in the state in which it is disconnected from the battery. The capacitor used can be that of the intermediate circuit of an inverter. The absence of a charge current across the charging resistor is assessed as being an improper connection of the battery to the high-voltage system.